Buti, an 84-year-old government-appointed cleric reviled by the Syrian opposition movement, delivered the official weekly Friday mosque sermons on state television.

In one of his televised speeches, Buti described those fighting to topple president Bashar al-Assad as "scum"'.

He also used his position to call on Syrians to join the armed forces and help Mr Assad defeat his rivals in the rebellion.

Buti's death will be a blow to the regime of Mr Assad, which has been fighting an insurgency that flared when his forces launched a bloody crackdown on Arab Spring-inspired democracy protests that erupted in March 2011.

The relentless violence has killed tens of thousands of people and caused more than 1 million to flee their homes.

Pro-government television aired gruesome footage from inside the mosque, where dozens of corpses and body parts were strewn on the carpeted floor among pools of blood.

Emergency workers collected the remains and carried them out in grey body bags.

Syrian Opposition Council president Ahmed Moaz Al-Khati said the opposition "categorically condemn the assassination".

"This is a crime, by any measure, that is completely rejected," he the AFP news agency.

Mr Khatib, himself a cleric, said he had known Mr Bouti, who was held in high esteem among Islamic theologians, but had disagreed with him over his vociferous support for Assad.

Born in 1929, Mr Bouti was from a large Kurdish family and spent years studying Islam, including at Cairo's al-Azhar University.

He was reviled by the opposition and frequently lashed out against the rebels while encouraging Syrians to join the army to fight against them.

The regime is dominated by members of the minority Alawite sect and had relied in part on the cleric to bolster its claims to represent all Syrians, including Sunnis, from whom the opposition draws much of its support.